Previously, seismic systems have been used to detect the presence of both heavy and light ground based vehicles. However, seismic detectors have some inherent problems. One of these problems is with wind induced seismic detections. The seismic alert system would give a false signal if the wind would be in the area of 10 to 30 mph. Further, seismic systems are also dependent upon the energy level of the seismic signal and not the frequency of the signal.
An example of a previous seismic device is U.S. Pat. No. 4,415,979, by Hernandez. The patent teaches a seismic sensor which utilizes an analog signal to calculate power. U.S. Pat. No. 4,604,738, by Aggerwal et al. with the same assignee as this invention, classifies a vehicle as to a light or heavy according to the ratio of the Rayleigh seismic signal and the acoustically coupled seismic signal. Thus, this patent requires both the use of a microphone and a geophone. Other seismic signal detectors have been identified and will be included in the disclosure statement accompanying this application.
The second method often used to detect the presence of light and heavy vehicles has been acoustic signal processors. The present acoustic signal processors identified in prior art have been concerned with two things, they are either analog signal processors, or they are concerned with the energy level of the incoming signal. The applicant's invention utilizes an acoustic signal processor, however, the signal processing is digital and is concerned with the frequency of the incoming signal.